The present invention relates generally to an illumination optical system, and more particularly to an illumination optical system, exposure apparatus and device manufacturing method for exposing an object, such as a single crystal substrate for a semiconductor wafer, and a glass plate for a liquid crystal panel.
The recent demands for smaller and thinner profile electronic devices have increasingly demanded finer devices, such as a semiconductor device, to be mounted onto these electronic devices. A projection exposure apparatus is a vital apparatus to the photolithography process that is generally used to manufacture these devices, and exposes a pattern of a reticle (or mask) onto a resist-applied substrate, such as a silicon wafer and a glass plate.
The resolution R of the projection exposure apparatus is given by the following equation, where λ is a light-source wavelength λ, NA is a numerical aperture of the projection optical system, and k1 is a process constant of a development process:
                    R        =                              k            1                    ×                      λ                          N              ⁢                                                          ⁢              A                                                          [                  EQUATION          ⁢                                          ⁢          1                ]            
Equation 1 indicates that a smaller wavelength and a larger NA improve the resolution. However, the smaller wavelength would disadvantageously reduce the transmittance of a glass material, the depth of focus (“DOF”) decreases in inverse proportion to the NA, and a larger NA makes difficult a design and manufacture of a lens.
Accordingly, the resolution enhanced technology (“RET”) has recently been proposed which reduces the process constant k1 for the fine processing. One RET is a modified illumination, which is also referred to as an oblique incidence illumination or an off-axis illumination. The modified illumination arranges an aperture stop with a light shielding plate on an optical axis in an optical system near an exit surface of an optical integrator for forming a uniform surface light source, and introduces exposure light oblique to the reticle. The modified illumination includes an annular illumination and a quadrupole illumination in accordance with a shape of the aperture stop.
It has been conventionally proposed to configure the modified illumination with only the linearly polarized light in a desired direction so as to improve the image contrast. See, for example, Japanese Patent Applications, Publication Nos. 7-183201 and 6-053120. Japanese Patent Application, Publication No. 7-183201 filters out the light different from the desired polarization direction. Japanese Patent Application, Publication No. 6-053120 discloses a method of using a linearly polarizer to generate a linearly polarized light in advance and of arranging a λ/2 phase plate to generate a linearly polarized light in a desired direction.
Other prior art include Japanese Patent Applications, Publication Nos. 2001-284212, 2001-284237, 6-275493, and 11-176721, and Japanese Patent No. 3,246,615.
However, Japanese Patent Application, Publication No. 7-183201 removes the light other than the light of a desired polarization direction, deteriorates the illumination efficiency and lowers the throughput. Japanese Patent Application, Publication No. 6-053120 causes the light that passes the λ/2 phase plate to be the elliptically polarized light instead of the linearly polarized light when there is a phase offset caused by an optical element in the illumination optical system, problematically lowering the image contrast.